In heavy duty manufacturing environments it is often necessary to handle, e.g., lift, support, and transport, heavy loads by an overhead traveling crane provided with some type of load engaging lifting apparatus. For example, in the manufacture of steel, molten steel from a basic oxygen furnace is cast into ingots either individually or by a continuous casting process. The ingots are then forged into slabs, blooms or billets depending on the final product to be manufactured, each ingot being a heavy load to be handled by an overhead traveling crane. The slabs, blooms, and billets form heavy loads that, at one time or another, must be lifted, supported, and transported from one location of a steel mill to another for further processing. For example, slabs manufactured by a continuous casting line are usually conveyed by a conveyer roller table to a piler delivery table, and piled in a slab yard by an overhead traveling crane having a load engaging apparatus in the form of a slab grip lifter. The slabs may be placed in the slab yard for storage or may be placed on a vehicle to be transported to another location, e.g., a furnace, another steel mill, and the like. An operator or a computer-controlled loader may load and unload the slabs to and from the slab grip lifter. This process may be conducted by an operator or may be automatic.
A conventional load engaging lifter apparatus most often employs one or more pairs of tongs having scissors-like linkages and jaws for grasping single or stacked heavy loads such as the steel slabs, blooms, and billets described above. Each set of tongs may be equipped with two or more pairs of arms for lifting and handling the heavy loads. Conventional load-engaging lifting apparatuses also include one or more bails for receiving a crane hook of an overhead traveling crane. When the tongs are lowered onto a load, the jaws are placed in a fully opened position, e.g., they are opened in an outward direction away from the load so as to provide an adequate opening for receiving the load. When the load is ready to be lifted, the jaws are closed inwardly, e.g., towards the load, until they contact the load. The overhead traveling crane then lifts the load engaging lifter upwards by means of the crane hook(s) placed through the bail(s). The upward movement of the lifter in combination with the weight of the load collapses the scissors like linkages of the lifter and thus draws together the jaws of the tongs in a fully clamped position against the load. The gripping power is generally mechanical and self-pressing against the load according to the weight of the load being lifted. Once the load is fully engaged in this manner, it is ready to be safely lifted, supported, and transported by the overhead traveling crane to a desired location.
There also exist motorized lifters for handling heavy loads. In such motorized systems, a motorized hoist is used to open and close the jaws of the one or more tongs of the load engaging lifter. Once the motorized hoist closes the lifter's jaws against the load, the gripping power is mechanical and self-pressing against the load according to the weight of the load.
The motorized lifters generally employ an electric motor that may be a DC or AC type motor for opening and closing the tongs. The conventional electric motors, however, suffer several disadvantages. A DC motor includes a commutator with brushes to supply electrical current. Brushes require maintenance and can lead to higher maintenance and repair costs. Furthermore, a DC motor has a limited dynamic response due to line commutation restrictions, coupled with higher mass moments of inertia imposed by the wound field armature. Also, commutation restrictions further limit the range of horsepower in which a DC motor can operate. Additionally, in a DC motor there always exist a potential for rapid acceleration to destructive velocities upon the loss of the stationary field.